Filtration is effectively a mechanical barrier that retains the solids while allowing the fluid (liquid or gas) to pass through. The application of solid-liquid and solid-gas separation technologies is ubiquitous in consumer products, as well as the agricultural, commercial, industrial, military and residential markets. Finally, an efficient solid-liquid separation technology is critical to pollution control, and to make water a sustainable resource to support population growth and promote industrial development.
A wide range of filter designs have been invented over the centuries for the separation of solids from a fluid (liquid or gas). The solids could be valuable products to be recovered or impurities to be removed from the fluid. There are all types of filter design available on the market, from a simple sieve design using a woven fiber cloth to advanced nano materials such as graphene; and from a simple disposable mechanical barrier and filter housing, to a complex system consisting of sensors, valves and logic controller for performance monitoring, and when required, initiate a series of back flushes and/or chemical clean-in-place processes.
All solid-fluid separation filters are based on three principal design parameters: 1) types of mechanical barrier, 2) motive force and 3) filtration surface area. The effectiveness and therefore the type of filter design required is a function of the solid-fluid separation efficiency and productivity.
An example of a high productivity and low efficiency is the common house air filter. An air filter, using a low density woven fiber membrane may be inefficient at capturing all the particulate matters, but it is productive at filtering gross particles from a very large volume of air, as depicted for example in FIG. 1(a)—Surface Filtration, 100.
An example of a high efficiency and low productivity filtration technology is reverse osmosis (RO). A RO system is designed to remove most all the dissolved solids from the fluid stream using a membrane with very small pores, making it very efficient.
To minimize solids building up in the membrane surface, a cross flow design is employed, as depicted for example FIG. 1(b)—Cross Flow Filtration, 200; however, the membrane's productivity is low hence a very large amount of surface area must be provided, and a significant percentage of the fluid is rejected (20-40%) to minimize blinding.
To improve the air filter efficiency, a more tightly bound, pleaded membrane is employed, commonly referred to as an HEPA filter. In both cases, the accumulation of solids increases the differential pressure overtime across the mechanical barrier—membrane for RO and woven fiber for an air filter. Since the cost of the membrane is expensive, a chemical clean-in-place process is employed to periodically clean the fouled membrane. The cost of a woven fiber air filter is relatively low, hence it is more practical and cost effective to replace and dispose of the used filter.
The common problem with all filtration devices on the market today is the accumulation of solids as the fluid flows tangentially across a static mechanical barrier, trapping the solids on the surface and subsequently must be cleaned or the filter media replaced. The accumulating solids on the filter media increases the pressure gradient across the mechanical barrier regardless of the types of mechanical barrier and motive force used in a filtration system.
The filter life can be extended by: increasing the surface area available for a static type screen filter (pleated cartridge and/or multiple cartridges for a solid-liquid system, or pleated HEPA filter and/or multiple chambers for a solid-gas system); mechanical compression of the solids on the filter housing and then dislodging the cake for a solid-liquid system (Fundabac®) or back pressure pulsing of the baghouse filter to dislodge the solids for a solid-gas system; physical removal of the accumulating solid cake on the filter media using a blade; back washing the media or multi-media filter; rejecting the concentrated filtration stream (reverse osmosis filter); or minimizing the accumulation of solids by imposing an high fluid velocity across filter media (cross-flow filter). The filter design available on the market today are based on a combination of these variations to remove the solids.